Powder, Electrode and Battery Comprising Such a Powder

ABSTRACT

Powder comprising particles comprising a matrix material and silicon-based domains dispersed in this matrix material, whereby either part of the silicon-based domains are present in the form of agglomerates of silicon-based domains whereby at least 98% of these agglomerates have a maximum size of 3 μm or less, or the silicon-based domains are not at all agglomerated into agglomerates.

The present invention relates to a powder, more specifically for use,either or not after further processing, in an electrode of a battery,and to an electrode and a battery comprising such a powder.

BACKGROUND

Lithium ion (Li-ion) batteries are currently the best performingbatteries and already became the standard for portable electronicdevices. In addition, these batteries already penetrated and rapidlygain ground in other industries such as automotive and electricalstorage. Enabling advantages of such batteries are a high energy densitycombined with a good power performance.

A Li-ion battery typically contains a number of so-called Li-ion cells,which in turn contain a positive (cathode) electrode, a negative (anode)electrode and a separator which are immersed in an electrolyte. The mostfrequently used Li-ion cells for portable applications are developedusing electrochemically active materials such as lithium cobalt oxide orlithium nickel manganese cobalt oxide for the cathode and a natural orartificial graphite for the anode.

It is known that one of the important limitative factors influencing abattery's performance and in particular battery's energy density is theactive material in the anode. Therefore, to improve the energy density,newer electrochemically active materials based on e.g. tin, aluminiumand silicon were investigated and developed during the last decades,such developments being mostly based on the principle of alloying saidactive material with Li during Li incorporation therein during use.

The best candidate seems to be silicon as theoretical capacities of 4200mAh/g (gravimetric) or 2200 mAh/cm³ (volumetric) can be obtained andthese capacities are far larger than that of graphite (372 mAh/g) butalso those of other candidates.

Note that throughout this document silicon is intended to mean theelement Si in its zerovalent state. The term Si will be used to indicatethe element Si regardless of its oxidation state, zerovalent oroxidised.

However, one drawback of using a silicon based electrochemically activematerial in an anode is its large volume expansion during charging,which is as high as 300% when the lithium ions are fully incorporated,e.g. by alloying or insertion, in the anode's active material—a processoften called lithiation. The large volume expansion of the silicon basedmaterials during Li incorporation may induce stresses in the silicon,which in turn could lead to a mechanical degradation of the siliconmaterial.

Repeated periodically during charging and discharging of the Li-ionbattery, the repetitive mechanical degradation of the siliconelectrochemically active material may reduce the life of a battery to anunacceptable level.

In an attempt to alleviate the deleterious effects of the volume changeof the silicon, many research studies showed that by reducing the sizeof the silicon material into submicron or nanosized silicon domains,typically with an average size smaller than 500 nm and preferablysmaller than 150 nm, and using these as the electrochemically activematerial may prove a viable solution.

In order to accommodate the volume change composite particles areusually used in which the silicon domains are mixed with a matrixmaterial, usually a carbon based material, but possibly also a siliconbased alloy or SiO₂.

Further, a negative effect of silicon is that a thick SEI, aSolid-Electrolyte Interface, may be formed on the anode. An SEI is acomplex reaction product of the electrolyte and lithium, and thereforeleads to a loss of lithium availability for electrochemical reactionsand therefore to a poor cycle performance, which is the capacity lossper charging-discharging cycle. A thick SEI may further increase theelectrical resistance of a battery and thereby limit the achievablecharging and discharging rates.

In principle the SEI formation is a self-terminating process that stopsas soon as a ‘passivation layer’ has formed on the silicon surface.However, because of the volume expansion of silicon, both silicon andthe SEI may be damaged during discharging (lithiation) and recharging(de-lithiation), thereby freeing new silicon surface and leading to anew onset of SEI formation

In the art, the above lithiation/de-lithiation mechanism is generallyquantified by a so-called coulombic efficiency, which is defined as aratio (in % for a charge-discharge cycle) between the energy removedfrom a battery during discharge compared with the energy used duringcharging. Most work on silicon-based anode materials is thereforefocused on improving said coulombic efficiency by means of reducing thevolume variations during charge—discharge cycling.

Current methods to make such silicon based composites are based onmixing the individual ingredients (e.g. silicon and the intended matrixmaterial or a precursor for the intended matrix material) duringpreparation of the electrode paste formulation, or by a separatecomposite manufacturing step that is then carried out via drymilling/mixing of silicon and host material (possible followed by afiring step), or via wet milling/mixing of silicon and host material(followed by removal of the wet medium and a possible firing step).

Known disclosures of methods for making composite powders comprising ahost material and a silicon-based powder dispersed therein are U.S. Pat.No. 8,124,279, U.S. Pat. No. 8,241,793 and U.S. Pat. No. 8,158,282. Forexample in U.S. Pat. No. 8,124,279 a composite of nanoscale siliconaggregate particles in a carbon-based material, is disclosed. Thecarbon-based material can be a mixture of a powder of particulategraphite having a mean particle diameter of from 1 μm to 100 μm; aconductive carbon black and a binder.

U.S. Pat. No. 8,062,556 also discloses a manufacturing method for thepreparation of a silicon-carbon nanocomposite wherein a silicon-basedpowder having particles with a size of less than 100 nm is mixed with acarbon containing polymer and subsequently pyrolized. The silicon-carbonnanocomposite is utilized as an active material in the preparation of anelectrode for Li-ion batteries, said electrode further including carbonblack and a binder.

Also in US2005/074672, EP1722429 and Xiang et al, CARBON 49 (2011)1787-1796 methods for preparing silicon based composites are disclosed.However, in these cases an amount of nano silicon powder was simplymixed with graphite. It is an achievement of the present invention torecognize that, mainly due to its nanometric size, such silicon powderswill strongly agglomerate into micron size agglomerates, which are noteasily broken up, at least not by standard processing steps.

Standard mixing, without specific measures to avoid these agglomeratesbreaking up, will therefore lead to agglomerates of nano siliconparticles in the final composite, which, as recognized by the presentinvention, are sub-optimal.

In US 2014/0255785 and US 2014/0234722 composites having individualsilicon particles are described. These silicon particles are embedded ina loose layer of graphene nanoplatelets or graphene sheets, leading to aporous structure with a high surface area.

This has the following disadvantages: The specific surface area, asindicated by a BET measurement, leads to excessive SEI formation.Further, the density will be low, leading to a low volumetric energystorage capacity.

Despite the advances in the art of negative electrodes andelectrochemically active materials contained therein, there is still aneed for yet better electrodes that have the ability to further optimizethe performance of Li-ion batteries. In particular, for mostapplications, negative electrodes having improved capacities andcoulombic efficiencies are desirable.

Therefore, the invention concerns a powder comprising particlescomprising a matrix material and silicon-based domains dispersed in thismatrix material, whereby either

-   -   part of the silicon-based domains are present in the form of        agglomerates of silicon-based domains and at least 98% of these        agglomerates have a maximum size of 3 μm or less, or    -   the silicon-based domains are not at all agglomerated into        agglomerates.

In other words, the silicon based domains and the matrix material form adispersion, so a system in which particles, in this case the siliconbased domains, are dispersed in a continuous phase of a differentcomposition or state, in this case the matrix material.

In other words either part of the silicon-based domains are present inthe form of agglomerates of silicon-based domains whereby d₉₈≦3 μm orthe silicon-based domains are not at all agglomerated,

Herein d₉₈ is the 98^(th) percentile of the distribution of the maximumsizes of the agglomerates.

By a silicon-based domain is meant a cluster of mainly silicon atomshaving a discrete boundary with the matrix. The silicon content in sucha silicon-based domain is usually 80 weight % or more, and preferably 90weight % or more.

The silicon based domains and agglomerates thereof may be observed viamicroscopic techniques of cross-sections of the particles of the powder.Via these microscopic techniques also the maximum size of agglomerates,if they are present, may be determined.

An agglomerate is a group of silicon based domains wherein the domainsare in contact, usually single point contact, with each other.

Such an agglomerate will usually be free or nearly free from matrixmaterial. Therefore the absence, or near absence, of matrix material inbetween silicon based domains positively identifies these domains asbelonging to a single agglomerate. The reverse is not necessarily true:the presence of matrix material is not sufficient to determine that agroup of silicon based domains is not a cluster.

For clarity it is remarked that the mentioned percentages concern thenumber of agglomerates with certain maximum sizes, not the weight thatthey represent.

If light microscopy or SEM techniques provide insufficient contrastbetween the silicon based domains, or agglomerates of these, and thematrix material, element mapping methods using EDX or EDS of thesecross-sections may be used, whereby low signals, or no signals at all,for elements indicative of the matrix material may be used to determinethe presence and size of agglomerates of silicon-based domains.

The maximum size of an agglomerate or domain is the largest measurablestraight-line distance between two points on the periphery of theagglomerate or domain.

In practice, such a silicon-based domain can be either a cluster ofmainly silicon atoms in a matrix made from different material or adiscrete silicon particle. A plurality of such silicon particles is asilicon powder, so that the silicon-based domains can be considered tobe a silicon powder.

The powder of the invention may be one of several types:

-   -   Carbon-based composite powder, in which a separately produced        silicon nano or submicron powder is agglomerated with separately        produced carbon and/or a carbon precursor acting as the matrix.        In this case the silicon domains are formed by the actual        discrete silicon particles from the silicon nano or submicron        powder.    -   So-called SiO_(x) particles, whereby overall x=typically        0.2-1.8. From investigations it has become apparent that this        overall formula consists of silicon domains in a SiO₂ matrix.        Such composite particles are formed by gas phase nucleation.    -   Silicon alloy, in which the other metal or metals in the alloy        or an intermetallic phase of the other metal or metals with        silicon form the matrix material and preferably are one or more        metals of the group consisting of Sn, Ti, Fe, Ni, Cu, Co and Al.        Such alloys are typically formed by alloying process, during        which submicron or nanometre sized silicon domains are formed.

The silicon-based domains may have a thin surface layer of siliconoxide.

The silicon-based domains may have any shape, e.g. substantiallyspherical but also whiskers, rods, plates, fibres and needles, etc.

For clarity it is remarked that that the silicon-based domains arenano-sized, having a mass-based average diameter d50 which is less than500 nm and preferably less than 150 nm. It is further noted that a smallsize of the silicon-based domains is considered as a boundary condition,without which a good composite cannot be produced.

Moreover, the composite powder itself comprises mainly micron-sizedparticles. It has a specific surface area as measured by the BETtechnique, of less than 10 m²/g, preferably of less than 5 m²/g and morepreferably of less than 2 m²/g

The composite powder according to the invention has a better cycleperformance than traditional powders. Without being bound by theory theinventors speculate that this is at least partly related to the factthat the powder according the invention will suffer less from the knownnegative effects of swelling and shrinkage of the silicon thantraditional powders, because the silicon is better dispersed. Thispositive effect is surprising because also in the traditional powderswhich have agglomerates of silicon based domains the expectation wouldbe that within an agglomerate there should be sufficient free space toallow expansion.

Further, such a powder according to the invention will have therebyindirectly have a strongly reduced tendency for SEI formation comparedto traditional powders with agglomerated silicon-based domains, and alsothereby gain in electrochemical performance.

In an preferred embodiment either part of the silicon-based domains arepresent in the form of agglomerates of silicon-based domains and atleast 98% of these agglomerates have a maximum size of 2 μm or less, andpreferably 1 μm or less , or the silicon-based domains are not at allagglomerated into agglomerates.

In a further preferred embodiment, the silicon-based domains are notagglomerated at all into agglomerates with a maximum size of more than 3μm and are preferably not agglomerated at all into agglomerates with amaximum size of more than 1 μm.

Possibly the matrix is carbon, e.g. thermally decomposed pitch or aprecursor material that can be converted into carbon by thermaltreatment, e.g. pitch. Such a powder has been shown to give goodperformance in a battery.

In a preferred embodiment the silicon-based domains are silicon-basedparticles, meaning that they were, before forming the composite,individually identifiable particles that existed separately from thematrix, so that they were not formed together with the matrix.

In another preferred embodiment the silicon-based particles arepreferably free from other elements than Si and O, so consist of siliconand oxidised Si, not taking inevitable impurities into account.

In yet another preferred embodiment the particles of the powderaccording to the invention contain only or nearly only said siliconbased domains and said matrix material, in other words contain at least90% by weight of said silicon based domains and said matrix material.

In yet a further embodiment the powder contains a carbonaceous material,preferably graphite, whereby the silicon-based domains are not embeddedin the carbonaceous material.

In an alternative embodiments the powder of the invention only or nearlyonly consists of said particles so that it contains at least 95% byweight of said particles.

The invention further concerns an electrode for an electrochemical cellcomprising the powder of the invention and a battery containing such anelectrode.

Preferably the composite powder contains between 2 weight % and 25weight % of silicon, and preferably between 8 weight % and 15 weight %of silicon. It preferably has an average particle diameter d50 ofbetween 1 and 20 microns.

The invention will be further explained by the following examples andcounterexamples, and illustrated by FIG. 1, which shows a SEM image of aparticle of a powder according the invention, whereby the white barrepresents 5 μm, and FIG. 2, which represents a part of the SEM image ofFIG. 1 on a larger scale, whereby the white bar represents 1 μmAnalytical methods used

Determination of Oxygen Content

The oxygen contents of the powders in the examples and thecounterexamples were determined by the following method, using a LecoTC600 oxygen-nitrogen analyzer.

A sample of the powder was put in a closed tin capsule that was putitself in a nickel basket. The basket was put in a graphite crucible andheated under helium as carrier gas to above 2000° C.

The sample thereby melts and oxygen reacts with the graphite from thecrucible to CO or CO2 gas. These gases are guided into an infraredmeasuring cell. The observed signal is recalculated to an oxygencontent.

Determination of Electrochemical Performance

All composite powders to be tested were sieved using a 45 μm sieve andmixed with carbon black, carbon fibres and sodium carboxymethylcellulose binder in water (2.5wt %). The ratio used was 90 weight partscomposite powder/3 weight parts carbon black/2 weight parts carbonfibres and 5 weight parts carboxymethyl cellulose (CMC).

These components were mixed in a Pulverisette 7 planetary ball mill intwo stages of 10 minutes at 500 rpm.

A copper foil cleaned with ethanol was used as current collector. A 125μm thick layer of the mixed components was coated on the copper foil.The coating was dried for 45 minutes in vacuum at 50° C. A 1.27 cm²circle was punched from the dried coated copper foil and used as anelectrode in a coin cell using lithium metal as counter electrode. Theelectrolyte was 1M LiPF₆ dissolved in EC/DEC 1/1+2%VC+10% FEC solvents.All samples were tested in a coin-cell tester with high precision(Maccor 4000 series).

The first discharge capacity and the coulombic efficiency of repeatedcharging and discharging cycles was determined. The coulombic efficiencyof the 9^(th) cycle is reported, as this is representative for theaverage between the 5^(th) and the 100^(th) cycle.

The skilled person will be aware that a small different in coulombicefficiency per cycle, will have, over the hundreds or thousands ofcharging-discharging cycles a battery is expected last, a significantcumulative effect.

Determination of Agglomerate Size

The maximum sizes of agglomerates of silicon particles was determined bySEM imagery by measuring the largest measurable distance between twopoints on the periphery of an agglomerate. Silicon and pitch, either asis or decomposed, could be easily distinguished visually, so siliconagglomerates could be easily identified by the prevalence of silicon,but especially by the absence of pitch.

The same procedure was repeated for the determination of agglomerateshaving a maximum size of below 0.5 μm, however the SEM micrographs weretaken with a higher magnification (preferably above 50.000×). To aid inthe counting and size measurement image analysis software was used. Toobtain reliable data at least 100 agglomerates were measured having amaximum size of at least 0.5 μm, if such agglomerates were present.

The samples were prepared according to well-known methodologies, e.g. byembedding them in resin followed by cutting and polishing to provide asmooth cross-section thereof.

EXAMPLE 1

A submicron-sized silicon powder was obtained by applying a 60 kW radiofrequency (RF) inductively coupled plasma (ICP), using argon as plasmagas, to which a micron-sized silicon powder precursor was injected at arate of 220 g/h, resulting in a prevalent (i.e. in the reaction zone)temperature above 2000K. In this first process step the precursor becametotally vaporized followed. In a second process step an argon flow wasused as quench gas immediately downstream of the reaction zone in orderto lower the temperature of the gas below 1600K, causing a nucleationinto metallic submicron silicon powder. Finally, a passivation step wasperformed at a temperature of 100° C. during 5 minutes by adding 100 l/hof a N₂/O₂ mixture containing 0.15 mole % oxygen. The gas flow rate forboth the plasma and quench gas was adjusted to obtain submicron siliconpowder with an average particle diameter d50 of 80 nm and a d₉₀ of 521nm. In the present case 2.5 Nm³/h Ar was used for the plasma and 10Nm³/h Ar was used as quench gas.

A blend was made of 16 g of the mentioned submicron silicon powder and32 g petroleum based pitch powder.

This was heated to 450° C. under N₂, so that the pitch melted, and,after a waiting period of 60 minutes, mixed for 30 minutes under highshear by means of a Cowles dissolver-type mixer operating at 1000 rpm.

The mixture of submicron silicon in pitch thus obtained was cooled underN₂ to room temperature and, once solidified, pulverized and sieved togive a powder with an average particle diameter d₅₀ of 17.8 μm.

A SEM microscopic evaluation was performed to determine if the siliconparticles in the silicon powder were agglomerated in the resultingcomposite powder. No agglomerates with a size of 0.5 μm or higher werefound.

The oxygen content of the powder was 0.95 weight %.

A SEM micrograph is shown in FIGS. 1 and 2, whereby it can be seen thatthe distribution of the silicon particle throughout the pitch was veryhomogeneous. In these pictures the white colour indicates siliconparticles and the dark colour indicates pitch.

Graphite (Showa Denko SCMG-AF) was added to the as-dried siliconpowder/pitch blend by dry-mixing, to obtain a siliconpowder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6,respectively.

10 g of the obtained mixture was fired in a quartz boat in a tubefurnace continuously flushed with argon and heated to 1000° C. at aheating rate of 3° C./min. The sample was kept at 1000° C. for 2 h. Theheating was turned off and the sample was allowed to cool to roomtemperature under argon atmosphere. The sample was removed from thequartz recipient, milled for 15 min in a coffee mill, and sieved toobtain a composite powder having an average particle diameter dm of 13.6μm. The oxygen content of the obtained composite powder was 0.8 weight%.

A SEM analysis was done to confirm that the size of the agglomerates hadnot grown due to the firing step. This was confirmed: No agglomerateswith a size of 0.5 μm or higher were observed. No porosity was visuallyobserved.

The specific surface area of the composite powder measured by the BETmethod was 1.8 m²/g

EXAMPLE 2

500 g of a submicron-sized silicon powder, obtained as in Example 1, wasmixed with 1000 g of petroleum based pitch powder.

In order to apply high shear, the blend was fed into a Haake process 11extruder, equipped with a twin screw and heated to 400° C., with thescrew running at a rotating speed of 150 rpm. The residence time in theextruder was 30 minutes.

The obtained extrudate, with silicon well dispersed in the pitchmaterial, was cooled down to less than 50° C. The injection port of theextruder and the container in which the extrudate was collected wereshielded from ambient air by flushing with N₂.

A part of the obtained extrudate was pulverized in a mortar, and sievedto give a powder with an average particle diameter dm of 15.9 μm.

A SEM microscopic evaluation was performed to determine if the siliconparticles in the silicon powder were agglomerated in the resultingcomposite powder. No agglomerates with a size of 0.5 μm or higher werefound.

The oxygen content of the powder was 0.98%.

Graphite (Showa Denko SCMG-AF) was added to the resulting siliconpowder/pitch blend by dry-mixing, to obtain a siliconpowder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6,respectively.

Hereafter, the obtained mixture was fired and sieved as described inExample 1.

The average particle diameter d₅₀ of the obtained powder was 14.1 μm andthe oxygen content was 0.79%

A SEM analysis was done to confirm that the size of the agglomerates hadnot grown due to the firing step. This was confirmed: No agglomerateswith a size of 0.5 μm or higher were observed. No porosity was visuallyobserved.

The specific surface area of the composite powder measured by the BETmethod was 3.7 m²/g

COMPARATIVE EXAMPLE 1

16 g of a submicron-sized silicon powder, obtained as in Example 1, wasdry-mixed with 32 g of petroleum based pitch powder.

This was heated to 450° C. under N₂, so that the pitch melted, and keptat this temperature for 60 minutes. No shear was applied.

The mixture of submicron silicon in pitch thus obtained was cooled underN₂ to room temperature and, once solidified, pulverized and sieved togive a composite powder with an average particle diameter dm of 11.2 μm.The oxygen content of the powder was 1.21%.

A SEM microscopic evaluation was performed to determine if the siliconparticles in the silicon powder were agglomerated in the resultingcomposite powder. The following results were obtained, with all resultsin μm:

Maximum size d10 d50 d90 d98 d99 observed 0.7 1.8 2.9 3.6 3.8 5.0

Graphite (Showa Denko SCMG-AF) was added to the resulting siliconpowder/pitch blend by dry-mixing, to obtain a siliconpowder/pitch/graphite mixture with a weight ratio of 1.0:2.0:7.6,respectively.

Hereafter, the obtained mixture was fired and sieved as described inExample 1. The average particle diameter d₅₀ of the obtained powder was16 μm, and the oxygen content was 0.9%

The SEM microscopic evaluation of the silicon particles and agglomerateswas repeated on the fired product. The following results were obtained,with all results in pm, showing that significant agglomeration of thesilicon nanoparticles had occurred:

Maximum size d10 d50 d90 d98 d99 observed 0.5 1.7 2.9 3.7 3.9 5.0

As can be seen the results are similar to the results on the unfiredproduct. SEM images showed porosity, especially between the siliconparticles making up an agglomerate of silicon particles.

The specific surface area measured by the BET method was 8.7 m²/g

The electrochemical performance was determined on all products afterfiring, and is reported in table 1.

TABLE 1 d98 of silicon d98 of silicon 1^(st) agglomerates agglomeratesdischarge Coulombic (μm) (μm) capacity efficiency at Product beforefiring after firing (mAh/g) cycle 9 (%) Example 1 <0.5 <0.5 645 99.46Example 2 <0.5 <0.5 646 99.51 Comparative 3.6 3.7 610 99.32 example 1

1-15. (canceled)
 16. A powder comprising particles comprising a matrixmaterial and silicon-based domains dispersed in the matrix material,wherein either a portion of the silicon-based domains are present in theform of agglomerates of the silicon-based domains and wherein at least98% of the agglomerates have a maximum size of 3 μm or less, or thesilicon-based domains are not at all agglomerated into agglomerates. 17.The powder according to claim 16, wherein the matrix material is acontinuous matrix.
 18. The powder according to claim 16, wherein atleast 98% of the agglomerates of silicon based domains have a maximumsize of 2 μm or less.
 19. The powder according to claim 16, wherein atleast 98% of the agglomerates of silicon based domains have a maximumsize of 1 μm or less.
 20. The powder according to claim 16, wherein allof the agglomerates of the silicon based domains have a maximum size of3 μm or less.
 21. The powder according to claim 16, wherein the powdercontains less than 3 weight % of oxygen.
 22. The powder according toclaim 16, wherein the powder contains between 2 weight % and 25 weight %of silicon.
 23. The powder according to claim 16, wherein the powder hasan average particle diameter d₅₀ of between 1 and 20 microns.
 24. Thepowder according to claim 16, wherein the silicon-based domains have amass-based average diameter d50 which is less than 500 nm.
 25. Thepowder according to claim 16, wherein the silicon-based domains aresilicon-based particles.
 26. The powder according to claim 16, whereinthe particles of the powder contain at least 90% by weight of saidsilicon-based domains and said matrix material.
 27. The powder accordingto claim 16, wherein the powder has a BET value of less than 10 m²/g.28. The powder according to claim 16, wherein that the particles have aporosity of less than 20 volume %.
 29. An electrode for anelectrochemical cell comprising the powder of claim
 16. 30. A batterycontaining the electrode of claim 29.